preinst is in python2.7-minimal 2.7.11-7ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39  | #!/bin/sh
set -e
syssite=/usr/lib/python2.7/site-packages
oldsite=/usr/lib/python2.7/old-site-packages
localsite=/usr/local/lib/python2.7/dist-packages
syslink=../../${localsite#/usr/*}
case "$1" in
    install)
	if [ -z "$2" ] && [ -d $syssite ] && [ ! -h $syssite ]; then
	    echo "new installation of python2.7-minimal; $syssite is a directory"
	    echo "which is expected a symlink to $localsite."
	    echo "please find the package shipping files in $syssite and"
	    echo "file a bug report to ship these in /usr/lib/python2.7/dist-packages instead"
	    echo "aborting installation of python2.7-minimal"
	    exit 1
	fi
	# remember newly installed runtime
	mkdir -p /var/lib/python
	touch /var/lib/python/python2.7_installed
    ;;
    upgrade)
    ;;
    abort-upgrade)
    ;;
    *)
        echo "preinst called with unknown argument \`$1'" >&2
        exit 1
    ;;
esac
exit 0
 |